I have had many types of dogs; Spaniel,Dachsund mix, Standard Poodles, Siberian Husky, German Shepard, &
Chihuahuas(I have 3 now).
I have to say the Standard Poodles were the smartest.
The Husky, the most gentle(my boys used to try to ride on her back)she never nipped, always running away though.
German Shepard,smart, but sheds quite a bit, big vet/food bills too.
Dachsund mix was best all-around dog.
Chihuahuas, most fun, since they are so small I can have 3.
I wonder if a Miniature Poodle would suit your needs. It's a medium size intelligent dog that doesn't shed much.